1. Field of the Invention
This invention relates primarily to methods and apparatus of fracturing subterranean formations using fracturing fluids that are hydrated from dry mix blends. In particular, one aspect of this invention comprises a dry blended particulate composition for hydraulic fracturing comprising a particulate hydratable polysaccharide, a particulate crosslinking agent, and a slowly releasing particulate base, using controlled release methods of particle dissolution. The invention reveals a dry blended particulate composition capable of significantly improved rheological properties at both low and high temperatures.
2. Description of the Prior Art
In the recovery of hydrocarbons from subterranean formations it is common practice, particularly in formations of low permeability, to fracture the hydrocarbon-bearing formation, providing flow channels. These flow channels allow the oil or gas to reach the wellbore so that the oil or gas may be pumped from the well.
In such fracturing operations, a fracturing fluid is hydraulically injected down a wellbore penetrating the subterranean formation and is forced against the formation strata by pressure. The formation strata or rock is forced to crack and fracture, and a proppant is placed in the fracture by movement of a viscous fluid containing proppant into the crack in the rock. The resulting fracture, with proppant in place, provides improved flow of the recoverable fluid, i.e., oil, gas, or water, into the wellbore.
Water-based hydraulic fracturing fluids typically comprise a thickened or gelled aqueous solution formed by metering and combining large volumes of fluids upon the surface of the ground, mixing them together in large mixing apparatus, and blending them with proppant before pumping the fracturing fluid mixture downhole. Proppant particles carried by the fracturing fluid remain in the fracture created, thus propping open the fracture when the fracturing pressure is released and the well is put in production. Suitable proppant materials include sand, sintered bauxite, or similar materials. The "propped" fracture provides a larger flow channel to the well bore through which an increased quantity of hydrocarbons can flow, thereby increasing the production rate of a well.
Obstacles facing the fracturing industry include large costs and environmental effects of operating and conducting fracturing treatments. Large costs are associated with storing and maintaining numerous liquids in large quantities in various, and sometimes remote, regions of the world. Further, the environmental effects of spillage and relatively large leftover quantities of fluid on site are increasingly becoming a problem for fracturing operators, as disposal of fluids is particularly troublesome under newer and more stringent environmental regulations.
Water-based hydraulic fracturing fluids usually contain a hydratable polymer that acts to thicken the fracturing fluid and may be further thickened by chemically crosslinking. Such a polymer typically is presented in a powder form, or in a slurried form in a hydrocarbon such as diesel, and is hydrated upon the surface of the ground in a batch mix liquid operation in large mixing tanks for a significant period of time, and then mixed with other liquid additives of various types using large expensive equipment. After hydration, the polymer is crosslinked to further thicken the fluid and improve its viscosity at elevated temperatures often encountered in the fracture, so it can carry proppant into the fracture once it is pumped into a wellbore below the ground surface. Natural polymers include polysaccharides, such as guar and derivatives of guar such as hydroxypropyl guar (HPG), carboxymethylhydroxypropyl guar (CMHPG), carboxymethyl guar (CMG), or hydrophobically modified guar. Borate, zirconium and titanium containing crosslinking agents typically are used. Both borate and organometallic crosslinking agents offer advantages depending upon the fluid performance and cost requirements of the particular fracturing treatment.
Numerous chemical additives such as antifoaming agents, acids or bases, or other chemicals may be added to provide appropriate properties to the fluid after it is hydrated.
It has long been recognized that large cost savings and convenience could be achieved by using a dry blend composition (i.e. similar in concept to a "cake mix") which is conveniently prepackaged for worldwide shipment, and which contains essentially all of the chemicals needed to prepare fracturing fluid in one dry granular packaged unit. Unfortunately, however, the granular compositions of the prior art have not provided the required storage stability and fluid properties needed in the industry, and have not offered the advantages that may be realized by this invention.
For example, U.S. Pat. No. 4,505,826 to Horton discloses a mixture of dry ingredients which, under some conditions, is stated to be capable of crosslinking at temperatures in the range of 80.degree. F. to about 130.degree. F. Zirconium acetyl acetonate is used as the crosslinking agent. The process, as set forth in the patent, apparently requires that the crosslinking agent become active before the gelling composition is completely hydrated. It is stated that if crosslinking of that particular fluid system is begun before the gelling composition is completely hydrated, further hydration is essentially halted and peak viscosity will never be reached, resulting in an inferior fluid.
Until the advent of this invention, it has been widely believed that hydration and crosslinking of a fracturing fluid composition could not occur simultaneously, because it was believed that no fracturing fluid system could achieve sufficient viscosity if it was "prematurely" crosslinked before the guar was fully and completely hydrated.
Other examples are known in which attempts were made to provide at least some of the fracturing fluid components in granular form. In 1974 and 1975, a fracturing fluid system was designed by Dustin Free, who was employed by a division of the Dow Chemical Company which was a predeccesor of the Dowell division of Schlumberger Technology Corporation, and his fluid is believed to have been used commercially after that time. That fluid system comprised liquid components and solid granular components. The mixture is believed to have been about: (1) 80 wt % guar, (2) a buffer having 3.3 wt % citric acid, 3) 6.66 wt % sodium acetate, (3) 8.0 wt % magnesium oxide, (4) 2 wt % silica flour, and was crosslinked with (5) liquid boric acid, wherein the liquid boric acid was added in "liquid add" form at the blender just prior to pumping the mixture downhole.
Other publications and patents have recognized the potential advantages of using a dry mix composition, but until the advent of this invention, the industry has not achieved a dry mix blend composition in which essentially all of the required chemical components (except of course the aqueous component) were placed in one dry mixture, the fluid being suitable for continuous mixing and simultaneous hydration and crosslinking of polysaccharide, with adequate storage stability. Further, until this invention, there was not a dry blend fluid system that was capable of providing the properties needed for optimum fracturing under downhole conditions.